THE solar system’s clockwork motion is by no means guaranteed: one day the Earth could collide with Venus or tear Mars apart in a close encounter, a new simulation has shown.

We know that the apparently reliable orbits of the planets are unstable in the long run, because their weak gravitational effects on one another can add up in unpredictable ways. Technically, the system is chaotic. Could this very mild chaos lead to disaster?

Mercury is the key to catastrophe. It is especially susceptible to Jupiter’s influence because of a small celestial coincidence: Mercury’s perihelion, the point where it gets closest to the sun, slowly moves around at a rate of about 1.5 degrees every 1000 years, and Jupiter’s perihelion moves around only a little slower. One day, the two will probably fall into sync, at which time Jupiter’s incessant gravitational tugs could accumulate and pull Mercury off course.

A study led last year by Jaques Laskar of Paris Observatory in France found a slim chance that Mercury’s orbit could be pulled into a highly elongated ellipse, putting it on a potential collision course with Venus. That work used a mathematical trick to calculate average changes over many planetary orbits, so the method was limited. "Close to a collision, it loses its validity," says Laskar. He and his colleague Mickaël Gastineau have taken a more thorough approach by directly simulating 2500 possible futures, calculating the planets’ orbits over 5 billion years, up to when the sun turns into a red giant.

Each of the 2500 cases has slightly different initial conditions - Mercury’s position varies by about 1 metre between one simulation and the next. In 20 cases, Mercury goes into a dangerous orbit and often ends up colliding with Venus or plunging into the sun. Moving in such a warped orbit, Mercury’s gravity is more likely to shake other planets out of their settled paths: in one simulated case its perturbations send Mars heading towards Earth.

Laskar found that Mars could hit Earth directly, be thrown out of the solar system, or come so close that Earth’s gravity would tear it into pieces which would rain down on our heads. Alternatively, the orbits of the inner planets could be scrambled, so Earth collides with Mercury or Venus.
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